How Bittorrent works: Data distribution simulator

This seems to be quite old already, but I just stumbled upon it a few days ago: The fine folks at Aphid.org have built a graphical simulator that shows how data is distributed when people are downloading a file through Bittorrent. Take a peek at an example:

It's a pretty simple simluation, but it clearly shows why Bittorrent works so well. The rarest parts get distributed first, and seeders distribute different parts to different leechers, thus making their upload more effective.